Begun in the autumn of the year 1845, without the cognizance, or at the suggestion of a single human being, the present Translation is due to the fact of its original having encountered a somewhat kindred spirit, and aroused therein the desire to render others participant, if possible, in the large amount of instruction it is so well calculated to afford. And now that the work is done, what remains for the labourer at second-hand to say by way of preamble to his newly-dressed wares? Had the book been printed within the pale of a philosophical or physico-theological sect, the Translator's final duty would have been clearly enough prescribed. Already bound to the profession of "particular tenets," his main object would be to indulge in a laudatory but servile abstract of his author's doctrines, or, if having set out with the expressed intention of illustrating their bearings upon the state of science past, present, and to come, he would become so drunk beforehand with the large and unbridled potation of his creed, as to surprise the casual reader by informing him that such an intention is useless, for the two stand in direct antithesis to each other. Examples of this mode of procedure are not wanting at the present day, whether at home or abroad. They are the produce of that spirit, which, rife enough in the Middle Ages, has been so graphically described by Professor Whewell under the title of the "Commentatorial," and "whose professed object is to explain, to enforce, to illustrate doctrines assumed to be true, but not to obtain additional truths or new generalizations." While from dealings of this character, as being utterly opposed to the sacred cause of Truth, I turn away with feelings of repugnance, to which the lessons of some personal experience have lent their aid, it is not my business, upon the other hand, to enter the lists of controversy against those who, having neither the capacity, nor the desire of its cultivation, for the higher walks of science, delight to dismiss a work of the present kind with some idle anathema of mysticism or evasive outcry for more facts.